1. Field of the Invention
This invention relates to an improved process and apparatus for the thermochemical beneficiation of carbonaceous materials, such as low rank coals, wherein the heating value of lump-size, low rank coal is chemically upgraded, its high inherent moisture content is reduced, mechanical handling characteristics are improved, and sodium content is reduced.
2. Description of the Prior Art
Deposits of low rank coals, such as lignite, subbituminous, and brown coal, represent one of the largest fossil fuel resources in the world. Most of the low rank coal deposits can be mined at a low cost relative to other fossil fuel sources. Low rank coals, in addition, have a very low sulfur content and thus when combusted, sulfur emission is very low.
Utilization of low rank coals and biomass materials, however, has been limited due to the high inherent moisture content of those coals. Low rank coals may contain about 30 to about 70 weight percent water. This factor combined with the typically low heating values of low rank coals has made the utilization of low rank coals as an energy source, economically unattractive.
One principal objective in the beneficiation of carbonaceous materials such as low rank coals and biomass materials, is to efficiently reduce their high inherent water content and to increase the heating value of the carbonaceous material. Air drying methods for carbonaceous materials, such as low rank coals, are not preferred because they have high energy requirements, they do not improve the heating value of the coal and the air dried coal readily decrepitates and reabsorbs moisture. Alternatives to air drying include steam treatment and hot water treatment of the carbonaceous materials, such as low rank coals.
The Fleissner process for steam drying lump-size low rank coals is described in U.S. Pat. Nos. 1,632,829 and 1,679,078. The objective was to produce a stable coal lump which could be easily and economically transported. The Fleissner process utilizes batch autoclaves which are operated sequentially to achieve preheating, steaming, depressurization, and air blowing. In practice, coal lumps up to 8 inches in diameter can be treated in this process, but removal, by preliminary screening, removes coal lumps smaller than 3/8 inch in diameter. Coal lumps finer than 3/8 inch, therefore, are not treated by this drying process and retain their high inherent moisture content and low heating value. Since the Fleissner process can only be operated as a batch process, it cannot economically be adapted for use in large scale operations.
Several other autoclaving treatments of coal are known. U.S. Pat. No. 3,552,031 discloses a process for separating moisture from organic materials wherein organic material is heated in an autoclave to temperatures between 100.degree. and 300.degree. C. in the presence of water at pressures greater than the saturation pressure. U.S. Pat. No. 3,007,259 teaches heating organic substrate with water and pressurized steam to about 200.degree. C. and then sequentially cooling the treated material. U.S. Pat. No. 2,966,400 teaches the use of a rotary preheating kiln in conjunction with a rotary processing kiln wherein temperatures are maintained at about 550.degree. to 650.degree. F., and steam reaches temperatures of 600.degree. to 1000.degree. F. A process for stabilizing lignite and subbituminous coal in which coal is dried to essentially zero moisture content and then partially rehydrated with steam is disclosed in U.S. Pat. No. 4,192,650.
The Koppelman process, described in U.S. Pat. No. 4,052,168, yields an upgraded product from low rank coal having a significantly reduced water content and an increased heating value. The Koppelman process uses a slurry system for feeding the coal and employs a conventional lockhopper system for discharging beneficiated product. This beneficiation process employs an autoclaving treatment which operates at about 1000.degree. F., at which temperature a significant portion of the coal is pyrolized. Due to pyrolysis, only about 55 percent of the dry coal feed is recovered as beneficiated product.
Several other slurry feed systems for beneficiating coal are known. U.S. Pat. Nos. 3,992,784 and 4,080,176 teach a process for thermal dewatering of brown coal which uses fine size low rank coals and employs a slurry feed system. The pumpable slurry is heated to temperatures above 150.degree. C. and pressure is maintained above the vapor pressure of water. A process developed in Finland known as the PDF Process for wet carbonizing peat also employs a slurry feed system to beneficiate only fine size low rank coals. U.S. Pat. No. 2,668,099 teaches a slurry system wherein a substantial portion of the heat requirement is supplied by subjecting the feed material, under pressure, to oxygen containing gas so that a portion of the material is oxidized to yield heat energy.
Because slurry systems require large volumes of water to be processed, complex and expensive heat exchange equipment is required to achieve high energy efficiency. Dewatering equipment costs are correspondingly high for the slurry systems. Moreover, transport of the beneficiated fine size low rank coal from the site of the beneficiation requires slurry pipelines or briquetting of the fine size coal for transport by rail or other means.
Hot water treatment is taught in U.S. Pat. No. 1,965,513 wherein fuel is immersed in water and heated to temperatures between 150.degree. and 250.degree. C. at the pressure of saturated water vapor.
U.S. Pat. No. 4,400,176 discloses a process of coal beneficiation wherein the inherent water content of the coal is reduced, at relatively low temperatures of about 220.degree. to about 500.degree. F., by contacting the coal with an acidic material, such as a carboxylic organic acid containing up to about 6 carbon atoms, phenol, phenolic acids or inorganic acids, during the thermal treatment. U.S. Pat. No. 4,014,104 teaches the addition of a non-aqueous solvent before heating and U.S. Pat. No. 2,610,115 discloses a method for heating solid fuel in the presence of a mineral hydrocarbon binder.
The process and apparatus of the present invention utilizes a thermochemical process of wet carbonization for beneficiating carbonaceous materials, such as low rank coals, including increasing the heating value and decreasing moisture, and which has several advantages over prior art processes. The process of this invention utilizes liquid sealed lockhoppers and can accommodate feed solids lump sizes up to about 4 inches in diameter. The process of this invention does not employ a slurry system which limits the solid particle size and requires complex heat recovery equipment. Beneficiation of carbonaceous low rank coals according to the process of this invention reduces the high inherent moisture content and increases the heating value of low rank coals, in addition to providing further advantages described below. Moreover, the wet carbonization beneficiation process of the present invention is energy efficient and economically superior to prior processes for use in large scale commercial operations.